This invention relates to a dual-gate Schottky barrier gate field effect transistor and to a method of manufacturing the same.
The dual-gate Schottky barrier field effect transistor ("Schottky barrier gate field effect transistor" will be briefly called "MESFET" hereinafter) is a semiconductor device having an ohmic source electrode, a first Schottky barrier gate electrode, a second Schottky barrier gate electrode, and an ohmic drain electrode disposed in series on an isolated n-type semiconductor substrate, and each section of the first and the second halves of the device is considered to form a single-gate MESFET. The device is commonly used as cathode type amplifier and in some cases as a modulator, a demodulator, or a mixer.
When the device is used as a cascode amplifier with power gain control at a low noise level, the noise figure or level of the device is mainly determined by the first section and the power gain is controlled by the second section, though both two sections contribute to the amplifying function. The device has not broadly been used as a cascode amplifier because the noise level of the device is not so low as is required. Another serious problem is that it is very difficult to produce a device having extremely short dimensions economically on a mass-production scale.
A proposal has been made by Asai et al. in "The 5th Conference on Solid State Devices, Tokyo (1973 International)", Japan Society of Applied Physics, Vol. 43 (1974), P. 442, to reduce the thickness of the semiconductor substrate under the first gate electrode for improvement of the electrical characteristics of the device. It seems difficult, however, to manufacture such devices on a mass-production scale because the electron beam lithography or the like which is not suitable for mass-production must be used as a technique for micro fabrication. Another proposal has been made by Ziel and Tankagi in "Improvement in the tetrode FET noise figure by neutralization and tuning, " IEEE J. Solid-State Circuits (Corresp.), Vol. SC-4 (June 1969), pp. 170-172, to provide an ohmic contact electrode connected to a neutralization circuit between the two gate electrodes for reducing the noise level. Formation of the neutralization circuit, however, becomes more difficult as the operating frequency rises to that of X-band.